1. Field of the Invention
The present invention relates to a zoom lens for projection composed of five lens groups, and a projection-type display apparatus on which the zoom lens for projection is mounted. In particular, the present invention relates to a zoom lens for projection and a projection-type display apparatus that are appropriate for magnified projection of a light beam output from a light bulb onto a screen. The light beam to be projected is output from the light bulb of a transmission-type or reflection-type liquid crystal display device, a DMD (digital micromirror device) display device or the like, and carries image or video information.
2. Description of the Related Art
In recent years, projection-type display devices, such as liquid crystal display devices and DMD display devices, which use light bulbs, became widely used. Especially, a projection-type display device using three light bulbs corresponding to illumination light of RGB primary colors to modulate the illumination light of respective colors are widely used. In the projection-type display device using three light bulbs, light modulated by the respective light bulbs is combined by a prism or the like, and an image is displayed on a screen through a projection lens.
A projection lens that is mounted on the above-mentioned projection-type display device, which combines modulation light output from three light bulbs by using a color combination optical system, requires long back focus to arrange a prism or the like for combining colors, and to prevent thermal problems or the like. Further, the spectrum characteristics of the color combination optical system change depending on the incident angle of light entering the color combination optical system. Therefore, the projection lens needs to have a characteristic that the entrance pupil viewed from the reduction side is located sufficiently far, in other words, the projection lens needs to be telecentric. Further, it is necessary that the projection lens has a low F-number, and that aberrations are corrected appropriately for the resolution of the light bulbs.
Further, a zoom lens, which can change the size of an image projected onto the screen, is often used as the projection lens. Recently, a zoom lens that can change the size of the image at a high ratio, in other words, a zoom lens with a high zoom ratio is needed. Further, as the price of a projector has become lower, the cost of the projection lens needs to be reduced.
Conventionally, a zoom lens composed of five lens groups, three of which move when magnification is changed, and the small conjugate length side of the zoom lens being substantially telecentric is, disclosed, for example, in U.S. Pat. No. 7,522,348 (Patent Document 1), Specification of Japanese Patent Application No. 2008-293932 (Patent Document 2) and Specification of Japanese Patent Application No. 2008-312145 (Patent Document 3).
The zoom lens disclosed in Patent Document 1 has a low F-number. Although the total number of lenses in Patent Document 1 is small, various aberrations are corrected in a relatively excellent manner. However, the zoom ratio of the zoom lens disclosed in Patent Document 1 is only 1.2 times. If the zoom ratio is increased without changing the structure of the zoom lens disclosed in Patent Document 1, variation of aberrations increases, and it becomes impossible to maintain the good performance of the lens.
Meanwhile, the zoom lenses disclosed in Patent Documents 2 and 3 have low F-numbers. Further, the zoom ratios are approximately 1.6 times, which is high, and various aberrations are corrected in a relatively excellent manner in spite of the high zoom ratio. However, the total number of lenses is 13, which is many. Further, since anomalous dispersion glass is used as the glass material of the zoom lenses, the need for reducing the production cost of zoom lenses is not satisfied.